The Novato Quarry terrane

Here’s a deeper look at the local bedrock. The hill on which Piedmont and its surrounding neighborhoods sit is held up by a body of Franciscan rocks, roughly 100-150 million years old. It’s mostly sandstone, as shown in the Dracena Park quarry pit, and mélange, as shown in the Mountain View Cemetery grounds. But the huge Franciscan Complex consists of a subset of nine different bedrock packages or terranes. Oakland’s Franciscan inventory is part of the Novato Quarry terrane, named for (of course) its exposure in a quarry in Novato. But here in Oakland is its southern end (I’m pretty sure). The terrane stretches from here to Bodega Head in a discontinuous string across the structural grain of the land. This view (click full size), from Dwight Canyon, shows other outposts of the Novato Quarry terrane, with Albany Hill (“el cerrito”) in front, then Point Richmond, and at rear right the rocks of Novato itself and Big Rock Ridge beyond. Another locality is Point San Pablo. To the west of it is the Marin Headlands terrane, which is self-explanatory.

What strung out the rocks of these terranes was movements on the various parts of the San Andreas fault system, which has sliced and spread out the pre-existing geology that was compiled during hundreds of millions of years of straightforward subduction.

7 Responses to “The Novato Quarry terrane”

i started reading “geology of the san francisco bay region”, and i was wondering one thing about the rock maps. i think these only show the rock types at the surface, right? do we have any idea what the composition of the rocks are, say, 1000 feet down? 2500 feet down? 5000 feet down? or is it just too expensive to find out (because you’d have to take core samples all over the place?) or, is it just obvious from what’s poking up in different places plus knowledge of plate tectonics?

We have pretty good ideas of what’s under the surface. There is some drilling, for water or oil&gas wells and so on, but our main tools are geophysical methods. There are seismic profiles that use acoustic signals to map the subsurface. There are magnetic surveys, conducted from airplanes and satellites, that can outline buried rock bodies that match surface exposuresfor instance serpentinite has a strong magnetic signature. Gravity surveys similarly detect density variations due to changing rock types. And earthquake studies help us outline the shapes of active faults undergroundfault planes aren’t always flat. We can study the whole thickness of the crust with these tools. The picture is still blurry, but much better than knowing nothing.

I found the profile of the Hayward fault on that USGS page chilling. Somehow seeing the profile gave me a profound impression of the huge slabs of rock that are grinding past each other, with the fault the near-vertical, miles-deep edge of two plates. I had the feeling of looking over the edge of a cliff. What a guaranteed time bomb.

Great shot and explanation, Andrew. Albany Hill always was a puzzle that you’ve answered and more, revealing it as part of a hidden string of hills across the north Bay. Sweet!

although i’ve been exposed to this stuff when i was in college, reading “geography of the SF bay region” has kind of given me pause. our home here in the bay area seems so transient now. its hard to believe that just 20,000 years ago the edge of the pacific was way out in the farallons, and long before that all of this was underwater somewhere. not to mention that the bay itself has come and gone many times over the last couple of million years. kind of puts this recent round of climate change in perspective.

as for the hayward fault itself, i can only hang onto the slim hope that creep has relieved enough pressure such that we don’t have a massive earthquake anytime soon, but i know that’s just wishful thinking.